The present invention concerns an apparatus to transfer optical signals between a rotating part and a stationary part of a machine, particularly a computer tomograph in which an optical transmitter is arranged on a first of the two parts and an optical receiver with an optical detector is arranged on a second of the two parts, via which optical signals emitted by the optical transmitter are received. The optical receiver comprises an optical concentrator with an at least approximately horn-shaped geometry that concentrates incident optical radiation via internal reflection on side surfaces of the concentrator onto a detection area of the detector.
In many fields of technology today large data quantities are transferred between elements moving relative to on another at a slight distance, particularly individual apparatus parts of a measurement apparatus. The data are thereby frequently acquired with an adjustable apparatus part and, for further processing, must be transmitted during the data acquisition to an evaluation device on a stationary apparatus part. Examples of such an application include medical imaging, and particularly computer tomography, in which a large quantity of measurement data must be transmitted from a rotating part (what is known as the gantry) to the stationary part in real time during the rotation. The available transmission rate represents an important criterion for the data quantity that can be transmitted in real time.
In a computer tomograph, the measurement data are acquired via x-ray detectors attached to the gantry opposite an x-ray source and transmitted to the stationary part within which the gantry rotates during the measurement. The transmitted measurement data are then further processed for reconstruction of slice images of the examined body and shown to the operator. The data transmission between the rotating part and the stationary part normally ensues using a “data transmission ring”, which is concentrically mounted around the rotation axis, either on the stationary part or on the rotating part, and which during the rotation exhibits a slight separation from a transmitter or receiver attached on the opposite part. The measurement data are thereby transported via the data transmission ring. The coupling between the data transmission ring and the transmitter or receiver mounted on the part lying opposite is in many cases realized as a radio-frequency connection via capacitively coupled antennas.
Newer generations of computer tomographs are in the position to simultaneously acquire a plurality of slices of the examined body, such that much larger data quantities per time unit must be transferred by the increasing number of measurement channels for each additional slice. For particular applications such as with newer developments in heart computer tomography, the rotation speed of the gantry must be increased, resulting in the number of the data to be transmitted per time unit increasing.
For example, a known multi-slice computer tomography that simultaneously records at a rotation speed of 140 revolutions/min requires a data connection with a capacity of approximately 800 Mbaud. Via a further increase of the number of simultaneously acquired layers as well as of the rotation speed, transfer rates of up to in the range of 2.5–10 Gbaud can be achieved. With known radio-frequency transmission techniques, the data transmission with such a high transmission rate is problematic since the distance between the rotating part and the stationary part is comparable to a quarter of the wavelength. Shorter wavelengths lead to an increase of the costs for the maintenance of the mechanical precision and the adjustment of the individual components of the system. Furthermore, it becomes increasingly more difficult to deal with problems of the electromagnetic compatibility. For high transfer rates in computer tomographs, optical signal transmission techniques are therefore increasingly used. The presently known optical transmission techniques can be divided into four different concepts that are briefly explained in the following.
U.S. Pat. Nos. 4,996,435, 5,229,871 (“the '871 Patent”) and U.S. Pat. No. 5,469,488 (“the '488 Patent”) specify transmission systems in which the transmitter comprises a plurality of light sources that are attached to the rotating part. The light sources generate overlapping light rays modulated with the measurement data, these light rays being received by one or more detectors of the receiver on the stationary part.
To increase the light yield, in the '871 Patent, elliptical reflectors with a single or double curve are used that deflect the incident optical radiation onto the detectors. In the '488 Patent, the use of an optical concentrator is specified that concentrates the incident optical radiation onto a detection area of the detector via internal reflection on side surfaces. The concentrator exhibits an approximately horn-shaped geometry with planar side surfaces that are formed via metallically-coated resin plates. However, the production precision of such a concentrator is insufficient for many applications with higher transmission rates. The transmission technique used in these printed publications is additionally only applicable for low data rates, since the adaptation of the electrical delay between the many feed lines that lead to the light sources and/or receivers is difficult.
In a second known optical transmission technique, such as that known from, for example, German Patent Document DE 4421616 C2 or U.S. Pat. No. 6,043,916 (“the '916 Patent”), the modulated light of a light source arranged on a rotating part is laterally coupled into a ring of an optical fiber concentrically attached to the rotation axis on the stationary part. The light propagating in the fiber is received by a detector coupled at the axial end of the fiber.
The greatest difficulty in the realization of this technique exists in the lateral coupling of light into the fiber core with sufficient efficiency. Different solutions are proposed for this. In the case of the '916 Patent, a special fluorescent fiber material is used that is excited by the optical radiation emitted by the transmitter; the receiver detects the excited fluorescent radiation.
A second known possibility exists in the use of a specially produced synthetic fiber with small entrance windows into the fiber cladding which enable the lateral light entrance directly into the fiber core. The lateral light coupling efficiency is very good in this case. However, the entrance windows cause a very severe damping of the coupled light upon the propagation in the fiber. A problem common to both techniques exists in the coupling of the axially modulated light (escaping from the fiber) in the photodetector, which normally exhibits a smaller detector area than the exit area of the fiber core.
In a further known signal transmission technology such as is used, for example, in U.S. Pat. Nos. 5,535,033, 4,259,584 and 6,396,613, the light modulated with the signals is axially coupled at the rotating part into an optical fiber that is attached annularly and concentrically (relative to the rotation axis) to the rotating part. The optical fiber comprises a transparent fiber cladding and is thus modified such that it also laterally radiates the coupled light. A receiver arranged on the stationary part detects the light emitted by the fiber ring. The coupling efficiency for the coupling of the modulated light in the fiber is very good. However, the lateral emission effects a severe weakening of the signal along the fiber.
Different techniques are used to generate the lateral emission. In one technique, a synthetic fiber with a partially uncovered core is used, in another technique a synthetic fiber with integrated bubbles is used via which the light is laterally scattered. Due to the high losses of this transmission technique in which, respectively, light is emitted over the entire length of the fiber ring but is detected only at one location at which the receiver is directly located, the optical receiver must possess a high dynamic range as well as a large detection angle in order to improve the signal-to-noise ratio via the detection of an optimally large portion of the emitted light.
In a further known optical signal transmission technique, the stationary part and the part rotating at a slight distance from the stationary part are fashioned such that a light-reflecting, hollow light channel is formed on the inner surfaces between both parts. The light can thereby be coupled by the rotating part into the light channel and decoupled from the light channel at the stationary part. Different embodiments for the realization of this transmission technique are, for example, known from U.S. Pat. Nos. 4,555,631, 5,134,639, and U.S. patent Publication 2004/0062344.
In one of the known embodiments, the hollow light channel is formed by a first half that rotates with the gantry and a second half that is arranged on the stator of the gantry. In this embodiment, the light exiting from the light channel exhibits a high dispersion, due to mechanical tolerances and irregularities of the reflecting surfaces. Here as well, the photodetector must possess a large incident angle range and a large detection area in order to achieve a sufficient signal-to-noise ratio.
All previously known transmission techniques to transmit optical signals between a rotating part and a stationary part of a machine require an optimally efficient collection of the dispersed and scattered light for the consolidation on the small detection area of the receiver used. The use of microlenses is in many cases unsuited for this, since these are not in the position to focus incident light onto the receiver in different directions and with different propagation modes. Moreover, the effect of the mode hopping that occurs given the lasers used most in this field reduces the efficiency of the focusing of a lens.
The use of an optical detector with a large detection area does in fact enlarge the quantity of light that can be acquired, however leads to a loss in bandwidth. Furthermore, detectors with a large detection area are severely limited in terms of the maximum processable data rate. Thus, for example, an avalanche photodiode (APD) can only process up to approximately 1 Gbps. The cause for the limitation of the processable data rate lies primarily in parasitic electrical capacities of the photosensitive region that are proportional to the detection area. To increase the processable data rate, detectors with smaller detection areas must therefore be used that, however, in turn capture only a reduced light quantity.